The present invention relates generally to electro-mechanical switching systems and particularly to electronic systems for controlling transformer relays. In a preferred embodiment the transformer relay has at least two stable states which are dependent upon the direction of unidirectional current flow above a predetermined threshold in the secondary winding of the transformer relay.
There exist in the prior art various types of transformer relays. These transformer relays are generally capable of the remote switching of a load. The transformer relay has a primary winding and a secondary winding. A load switch is mechanically and magnetically coupled to the transformer relay core through the armature and the load is connected in series with the load switch while the primary winding of the transformer relay is coupled to a power source. The transformer relay has two stable states. One of the stable states is realized when the current flow in the secondary winding is restricted to a first direction and exceeds a predetermined threshold, and a second stable state is realized when the current flow in the secondary winding is restricted to the other direction and exceeds the predetermined threshold. Of course, the function of a transformer relay may be realized with the transformer and the relay actually physically constructed in separate physical packages. A transformer relay function is provided whenever a transformer and a relay are coupled in a manner in which the state of the relay is determined by the current flowing in the transformer.
One example of a prior art transformer relay is illustrated in the United States patent application Ser. No. 034,381, BAKER et al, LOW VOLTAGE TRANSFORMER RELAY, filed Apr. 30, 1979 and assigned to the assignee of the present application. The Baker transformer relay is of the type which has a unitized transformer and relay package. The relay is magnetically latched and directional current flowing above a predetermined threshold in the secondary winding controls the position of the load switch. Generally, the voltage and current present in the secondary winding are less than the voltage and current which are available at the power source to which the primary winding is connected. In a typical case, the primary winding of the transformer relay may be connected to a 120 volt alternating current source capable of supplying 15 to 20 amperes, while the secondary winding of the transformer relay may supply approximately 15 volts and around 100 milliamperes of current.
An example of a switching system to control a transformer relay is illustrated in U.S. Pat. No. 3,461,354, Bollmeier, issued Aug. 12, 1969 and assigned to the assignee of the present application. The switching mechanism disclosed for controlling the transformer relay in the Bollmeier patent uses a double pole, double throw switch with the common terminals of the double throw switch coupled to each side of the secondary winding. A diode is coupled across two switched terminals of the double throw switch which are in turn cross-coupled to the other switched terminals to allow selective directional current flow in the secondary winding depending upon the position of the double pole, double throw switch. Similar results may be obtained with a single pole, double throw switch with common terminal coupled to one side of the secondary winding and the switched terminals coupled to the other side of the secondary winding through diodes of opposite polarity. An example of this latter arrangement is illustrated in the Baker et al patent application.
Many conditions exist where there is a need to control the transformer relay secondary winding with a control circuit or control device which is insufficient to meet the control criteria of the secondary winding of the transformer relay. That is, not only must the directional current be established in the secondary winding in order to switch the transformer relay, but a directional current flow of sufficient amplitude to switch the magnetic structure, i.e. a directional current above a predetermined threshold, must be obtained.
For purposes of safety, certain wet locations need a very limited voltage and current. That it, if a location is determined to be wet, then the maximum amount of voltage allowed to be present at that location as well as the maximum amount of current allowed to be drawn at that location must be limited if that location is to remain absolutely safe. Examples of these wet locations include, but certainly are not limited to, hot tubs, bathrooms, and certain outdoor locations.
Another condition exists where the control to the secondary winding of the transformer relay is insufficient. This occurs where either the voltage or the drive capability of the control device is insufficient to operate the transformer relay. This could occur very easily where it would be desired to have a computer or a microprocessor operate the transformer relay or a whole series of transformer relays.
Another case of insufficient control to the transformer relay exists where the control device is simply incompatable with the transformer relay secondary. An example of this kind of incompatibility would be in the use of timers or sensors to control the transformer relay. Such sensors could be derived from a number of external stimula, including light, infrared, thermal, ultrasonic and microwave.
Another example of incompatibility is where the control desired is merely a unidirectional control. Examples of unidirectional control would be blackout protectors to switch off a particular piece of equipment, should power be interrupted, to protect it from voltage or current surges should power suddenly be reapplied, and any equipment which needs to be preset, either to an "on" or an "off" condition as power is applied. A control circuit may be desired to assure that the transformer relay would energize into the appropriate state.
Another example of the insufficiency of the control media to interface directly to the transformer relay, is the inability to adapt to certain standard controls which already exist. An example is the switching of a lighting or equipment load with a transformer relay using a standard single pole, single throw wall switch as a control device. Note that the Bollmeier control requires a double throw switch in this installation, and thus cannot be used in existing applications without modification of the switch.
Still another example of the insufficiency of the control media is related to the use of a multiplicity of control devices. The problem is that the total leakage current through the control devices must be kept below the minimum switching level, threshold, of the transformer relay secondary winding. Although the leakage current through a single control device may be well below that amount of current which would switch the transformer relay, the connection of a large plurality of controls may cause that leakage current to surpass the minimum switching threshold. The problem is especially apparent where the control device itself is drawing its power from the secondary winding of the transformer relay, a common and desirable situation.
Still another example of insufficient control of the transformer relay is the case where a plurality of control devices are utilized to control one transformer relay and those control devices are of different types; perhaps requiring differing interface characteristics. For example, it may be desirable to utilize automatic timers along with manual wall switches to control a lighting situation. Another example would be where a given load may be controlled from switches, some of which are located in dry locations and some of which are located in wet locations. In this instance, it would be necessary to install a safe switch in the wet locations, but certainly not necessary to install such a safe switch in the dry locations. Still another example of differing types of controls to be utilized with one transformer relay is a computer control also coupled with a blackout circuit protector. In this situation, the computer will have normal control over the operation of the load while a blackout protector may also be connected to the transformer relay to ensure that the load being controlled is protected in the case of a power interruption.